Delphi XE2 - Inheriting class not calling base class' constructor...?

Question

When creating a class that inherits from another class, shouldn't it be true that when the derived class is created the base classes's constructor is called?

Type
  TBase = Class
    constructor xMain;
  End;
  TDerived  = Class(TBase)
    constructor xMain;
  End;

constructor TBase.xMain;
begin
  MessageBox(0,'TBase','TBase',0);
end;

constructor TDerived.xMain;
begin
  MessageBox(0,'TDerived','TDerived',0);
end;


Var
  xTClass:TDerived;
begin
  xTClass := TDerived.xMain;
end.

I thought this should result in a MessageBox displaying "TBase" and then "TDerived". Yet, this is not the case. When the above code is ran it only results in one MessageBox displaying "TDerived".

You should name constructors Create even if the language itself does not force you to do so. — Stefan Glienke, Nov 14 '12 at 07:12

score 9 · Accepted Answer · answered Nov 11 '12 at 10:26

9

constructor TDerived.xMain;
begin
  inherited;
  MessageBox(0,'TDerived','TDerived',0);
end;

answered Nov 11 '12 at 10:26

bummi

27,123
14
62
101

Well that's embarrassing. Thanks. Will accept in few mins when it lets me. – Josh Line Nov 11 '12 at 10:30

score 6 · Answer 2 · edited May 23 '17 at 12:08

6

add inherited in TDerived.xMain; otherwise the code from ancestor will not be called;

begin
  inherited;//call the ancestor TBase.xMain
  MessageBox(0,'TDerived','TDerived',0);
end;

Also this question will help you understand inherited reserved word:

Delphi: How to call inherited inherited ancestor on a virtual method?

another good resource is http://www.delphibasics.co.uk/RTL.asp?Name=Inherited

edited May 23 '17 at 12:08

Community

1
1

answered Nov 11 '12 at 10:31

RBA

12,337
16
79
126

You also may want/need to call inherited for overridden methods - IF you want to perform the ancestor methods functionality and then add to it. – Toby Dec 30 '16 at 17:40

Delphi XE2 - Inheriting class not calling base class' constructor...?

2 Answers2